Method and system for cleaning membrane filters

ABSTRACT

The disclosure relates to a system and method for cleaning filters, such as membrane filters. More particularly, a method and system are disclosed for retaining a plurality of small particulates, preferably in the shape of beads, which contact sludge or other despots on the membrane filters to remove unwanted debris that would otherwise form on the cleaning filters. In various embodiments, the plurality of small particulates are retained in a permeable enclosure formed of wedgewire.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/576,662, entitled “Method and System for Cleaning MembraneFilters,” filed on Dec. 16, 2011, which is incorporated by referenceherein in its entirety.

FIELD OF THE INVENTION

The present invention is directed to the cleaning of membrane filters,and more particularly, to a method and system that retains a pluralityof small particulates, such as particulates generally in the shape ofbeads, that contact membrane filters while in a solution so as to removedebris that would otherwise form on said filters, with said plurality ofsmall particulates being retained in a permeable enclosure.

BACKGROUND

Membrane water filtration is well known and is increasingly popular dueto its extreme efficiency in clarifying water and removing undesiredcontaminants and components typically encountered in municipal watertreatment facilities. A particular membrane filtration system is offeredby Microdyn-Nadir GmbH. U.S. Pat. No. 7,892,430, U.S. Patent ApplicationPublication Nos. 2011/0042308, 2011/0042312, 2011/0049038, 2011/0127206,and International Application PCT/EP2009/002944 are incorporated hereinby this reference in their entireties in order to provide support forthe basic membrane filtration technologies involved in practicing thebest mode of the present invention.

A particular problem encountered by use of membrane filters, however, isthe eventual build up of undesired debris and contaminates on thesurface of the membrane filter. Conventionally, such debris andcontaminants must be removed through various means, including chemicalsoaking of the membranes in chlorine solutions. This necessitates takingthe filters out of use and commission during the cleaning process, whichcan last for a significant amount of time and thus impacts thecommercial use and nature of membrane filtration technologies. There istherefore a long felt, but unsolved, need for a method and system forcleaning membrane filters while such filters are in use performing theirwater filtration functions.

These systems, however, are known to cause fouling, discoloration, andgeneral deterioration of various filtration elements, which in turn havea negative impact on the water or wastewater application in which thefiltration elements reside. Furthermore, prior art systems often rely onchemical processes to address problems associated with the prior art,which have a negative environmental impact and may cause otherundesirable consequences. Additionally, such processes are known to beexpensive and require significant time and labor investments.

SUMMARY

The use of membrane bioreactors (MBR) and filtration membrane modulesfor treating raw water or wastewater is known in principle. Themembranes used for filtration consist, for example, of polymericmaterials such as polyethylene, polypropylene, polyethersulfone,polyvinylidene fluoride or similar polymers. The pore sizes of themembranes are for these uses in the range between 0.001 and 1 μm. In amembrane bioreactor (MBR), the activation method for wastewatertreatment with separation of the biomass from the purified water iscarried out using ultra- or microfiltration membranes. In mostapplications, the polymer membranes are immersed directly in theactivated sludge and the treated wastewater is drawn off by means ofvacuum suction or flows off under the influence of gravity.

In the MBR method, the wastewater is physically, chemically andbiologically treated in a plurality of steps until it reaches themembrane. By means of mechanical and physical pretreatments, thewastewater is freed from particles, fibers and coarse matter. In thecoarse filtration, large particles which could cause damage to themembranes are removed by grills and screens. In the MBR method, finescreens in a size range of 0.05-3 mm are customarily used asprefiltration. Additionally, the wastewater is freed from heavyparticles (e.g., sand) and oils and fats by a sand and fat trap.

In an embodiment, the use of an enclosure screen around the membraneeliminates or reduces the need for filtration in other treatment tanks.The enclosure screen may act as the final step to remove large particlesbefore the water reaches the filtration membrane. In another embodiment,the enclosure screen acts as protection for the membrane filter becauseit prevents large particles from reaching and damaging the membranefilter.

In a further treatment step of the wastewater treatment, the wastewateris biologically and chemically treated. In the activation tank there issituated the activated sludge (biomass) which contains in its biomassthe enzymatic potential for conversion of the high-molecular-weightpollutants in such a manner that these can be eliminated. The dissolvedmaterials are utilized by the biomass either for the cell structure orfor energy production with oxygen consumption. The resultant oxygenconsumption must be covered by sufficient oxygen supply, for whichreason activation tanks are provided with aeration appliances. Aprecondition for the functioning of the method is that the biomassremains in the system. Therefore, the biomass is separated off from thepurified wastewater by membrane filtration and recirculated to theactivation tank. Overgrown activated sludge is removed as excess sludge.Before the biomass is separated from the water, further chemicaltreatments proceed. In combination with a filtration stage, variousprecipitants and flocculants such as, for example, iron chloride orpolymers for removing colloidally and particulately dispersed liquidcomponents are customarily used.

A substantial advantage of MBR systems is the solids-free effluent. Thismeans, in addition, that no bacteria are found in the effluent of themembrane activation system and, even viruses may be separated off bysorption effects. The residual organic pollution is reduced therebyowing to the complete separation. The hygienically relevant guide valuesof the EU bathing water directive [75/160/EEC, 1975] are complied withusing MBR. In addition, the solids-free effluent offers not only in themunicipal sector but also in the industrial sector, a great potentialfor wastewater reuse. Here, by water recycling up to closed circulationof water large savings of water can be achieved. A further advantage isthat in this method, owing to the adjustable high DM content and theomission of the clarifiers, only a very small space is required. Owingto the independence of the sedimentation behavior, the activated sludgeconcentration (biomass concentration, expressed as DM—dry matter) can beincreased over conventional methods. Membrane bioreactors arecustomarily operated at DM concentrations of 8 to 15 g/l. Compared withthe conventional activation method, the reactor volume of a membranebioreactor can be reduced, in such a manner that higher volumetricloadings are possible.

In the case of the membrane bioreactor method which is based generallyon the aerobic activation method that is combined with a membranefiltration unit, the biomass is recirculated as concentrate over themembrane filtration unit, while the purified water is separated off asfiltration permeate.

A problem in the use of membrane filters in the field of wastewaterpurification is what is termed “membrane fouling”, which means thatdeposits form on the membranes, which deposits decrease the through-flowof the liquid that is to be purified.

DE 102 20 916 A1, which is hereby incorporated by reference in itsentirety, describes a filtration appliance and also a membranebioreactor which are operated under conditions in the filtration mediumsuch that membrane fouling and deposits on the membrane surfaces arereduced. For this purpose the filtration device has hollow fibermembranes that are combined to form a fiber bundle for separating offthe particles from a liquid, through which hollow fiber membranes liquidflows from the outside to the inside, and the filtered liquid is thentaken off from at least one of the ends of the hollow fiber membranes.The filtration device, in addition, has a gas feed device in order toflush a gas over the exterior of the hollow fiber membranes. The fiberbundle in this case is wound round the outer peripheral surface of acarrier of the gas feed device.

EP 1 734 011 A1, which is hereby incorporated by reference in itsentirety, discloses a method for improving the flow through a membranebioreactor, in which a certain fraction of cationic, amphoteric andzwitterionic polymers or a combination thereof is added. The fraction ofthe added polymers is 10 to 2000 ppm, based on the entire membranebioreactor volume. The polymers have a molecular weight of 10 000 to 20000 000 Da. Adding the abovementioned polymer should reduce, especiallyinorganic fouling, which is formed by the precipitation of limestoneCaCo₃ onto the membrane surfaces from the wastewater that is to bepurified. The pH increases in the course of this, whereby in turn theprecipitation is promoted by calcium phosphate and iron oxide. Theprecipitation of carbonates and phosphates in the wastewater proceeds inthe form of small particles which are retained on the membrane surfaces.

Quite generally it is true that membrane fouling due to theprecipitation of bioactive solids, colloids, accumulation of particlesor macromolecular particles on the membrane surface leads to a decreasein the through-flow and permeability. It is difficult to describe thefouling process exactly owing to the heterogeneity of the activatedsludge. Factors such as characteristics of the biomass, theextracellular polymeric substance, pore size, surface characteristicsand membrane material, and also the construction of the filter membranemodules and the operating conditions influence fouling growth. Forexample, biofouling occurs most frequently on nanofiltration and reverseosmosis. The reason is that the membranes cannot be disinfected withchlorine in order to kill bacteria. The biofouling is principally due tothe complex growth behavior of the bacteria. The type of microorganisms,the growth rate thereof and concentration on the membranes dependchiefly on the critical factors such as temperature, pH, theconcentration of dissolved oxygen and the presence of organic andinorganic nutrients. It should be noted that the microorganisms passinto the filtration systems via air and/or water.

In the case of the filtration methods using membrane bioreactors, thegrowth of the fouling is customarily monitored in a plurality of steps.

1. Pretreatment of the raw water or wastewater, before inflow into theactivated sludge, by means of various filtration steps as have alreadybeen mentioned above, for which purpose fine-mesh gratings having a meshwidth of 0.5 to 3 mm are used.

2. In the “crossflow” method, the liquid that is to be purified iscirculated along the membrane surface, for which purpose in the case ofsubmerged modules, aeration devices are installed below the membranemodules, which aeration devices induce an upward streaming.

3. In some membrane modules a regular fully automatic backwash withpermeate is performed, in such a manner that adhering particles/dirt aredetached from the membrane surface and the pores are flushed open. Aprecondition is that the respective membrane is backwashable.

4. Chemical cleaning: the steps serve to prevent membrane fouling or atleast decrease it. Chemical cleaning is necessary in order to remove themembrane fouling layers on and within the membranes. Chemical cleaninggives rise to considerable operating costs, since during the cleaningthe membranes are out of operation and therefore additional membranesmust be installed.

In addition, it is disadvantageous that the chemicals used such as, forexample, sodium hydrochlorite NaOCl adversely affect the environment andcontribute to the formation of absorbable organic halogen compounds(AOX). In addition, chemical cleaning requires an additionalinfrastructure (pumps, chemical containers, leak measuring devices,protective equipment, etc.) which is costly. Frequently, the membranesare chemically cleaned in a separate cleaning container in order to savechemicals, since these cleaning containers have small volumes. For thispurpose the membrane module must be taken out of the filtration pond ortank and installed in the cleaning pond or tank. In the cleaningpond/tank, the chemical cleaning then takes place. The operatingpersonnel must be trained to handle these chemicals and chemicalcleaning is labor-intensive. Overall, chemical cleaning is aconsiderable cost and environmental factor.

For avoiding fouling layers, the publication of the company VA TECHWABAG GmbH, Vienna, editor: F. Klegraf with the title “Beherrschung vonFouling and Scaling an getauchten Filtrationssystemen inMembranbelebungs-anlagen” [Managing fouling and scaling on submergedfiltration systems in membrane activation systems], which is herebyincorporated by reference in its entirety, describes the use ofabrasively acting inert inorganic porous materials which can detachdeposits on the surface of the membranes by long-term action. This useis not uncontroversial, since it must be feared that the abrasive forcesnot only erode the deposits, but also damage the sensitive surfaces ofthe membranes. As an inert abrasive material, expanded clay is mentionedwhich is introduced into the reactor. The expanded clay is retained inthe reactor by screens. The turbulence introduced into the reactor withthe flushing air is sufficient to homogenize the expanded clay in thesystem. Immediately after charging the reactor with expanded clay, theincrease in filtration performance can be measured and by carefulincreasing of the expanded clay concentration in the activated sludge,75% of the preset value of the filtration performance can be achievedafter an experimental time of 40 days. Further increase of the expandedclay concentration in the reactor is not accompanied by any lastingimprovement of the filtration results. The density of the porousexpanded clay increases with time owing to water absorption. Theexpanded clay particles become heavier thereby and settle within thereactor and circulate only to a small extent as a result of the liquidstreaming. In order to stimulate the circulation of the expanded clayparticles, relatively large amounts of compressed air are then necessarybut owing to the increased feed of compressed air into the liquids thatare to be purified, other process parameters can be adversely affectedthereby, for example maintaining preset theoretical oxygen values ismade considerably more difficult. The velocity of ascension of theparticles here is predetermined by the size of the air bubbles formed,but not by the amount of air introduced.

It is an object of the invention to provide conditions for a method forcleaning filtration membrane modules that are used in the treatment ofcrude water or wastewater or activated sludge in which the depositscaused by the membrane fouling are greatly reduced and mechanical damageto the membranes is substantially avoided. In the context of thisobject, the operating costs must also be reduced and flow of thewastewaters that are to be purified through the membranes must be keptconstant for a relatively long time.

This object is achieved according to the invention in that thefiltration membrane module that is to be cleaned is introduced into acleaning pond/tank or arranged in a filtration pond/tank, flushed by aliquid which contains non-porous biologically resistant particles andset in circulation with gas introduction and in that the depositsituated on outer surfaces of the membranes of the filtration membranemodule, termed membrane fouling, is mechanically eroded by theparticles.

According to one particular embodiment, a method for treating raw wateror wastewater or activated sludge may comprise one or more of thefollowing steps, which are in no particular order:

-   -   Mechanical, physical and chemical pretreatment of the raw water        or wastewater or activated sludge,    -   Introducing the raw water or wastewater contaminated with        biologically active material into a membrane bioreactor system        having one or more filtration tanks in which in each case at        least one submerged filtration membrane module is arranged,    -   Charging the raw water or wastewater or the activated sludge in        the filtration tank with particles circulating in the filtration        tank,    -   Taking off the water purified by the biologically active        material, and    -   As a special case in applications in which a separate cleaning        container is installed: installation of at least one membrane        module in the cleaning container, charging the cleaning        container with circulating particles.

In another embodiment of the method disclosed herein, the particles thatcirculate within the filtration tank perform an upward movement inducedby gas introduction, in particular by compressed air, and perform adownward movement effected by gravity. Expediently, the non-porousparticles consist of inert polymeric material that has a density of 1.0to 1.5 kg/dm³. The term “inert” is used here and hereinaftersynonymously with “biologically resistant” or not degradable by thebacteria in the activated sludge.

According to yet another embodiment of the present disclosure, a methodfor cleaning a filtration system is disclosed, comprising the steps of:

At least partially surrounding one or more filtration membrane modulesto be cleaned in a housing enclosure structure;

flushing the one or more filtration membrane modules with a liquidcontaining non-porous biologically resistant particles;

circulating the non-porous biologically resistant particles through theliquid and in a manner to achieve contact between the non-porousbiologically resistant particles and the one or more filtration membranemodules;

wherein deposits situated on one or more surfaces of the membranes ofthe one or more filtration membrane modules are mechanically abraded bythe particles, and wherein the particles are of a size sufficient enoughto be substantially retained within the housing enclosure structure.

According to yet another embodiment of the present disclosure, a methodfor cleaning a filtration system is disclosed, comprising a filtrationmembrane module having a housing enclosure structure surroundingfiltration membrane module, the enclosure structure having a multitudeof substantially regularly spaced orifices therein, said enclosurecontaining a plurality of non-porous biologically resistant particlesthat when set in circulation adjacent the filtration membrane module,deposits situated on outer surfaces of a membrane of the filtrationmembrane module are mechanically abraded by the particles, wherein theparticles are of a size larger than the orifices of the housingenclosure structure so as to restrain such particles within the housingenclosure structure.

The polymeric material is advantageously selected from the groupconsisting of polypropylene, mineral particle-containing, polycarbonateblends, thermoplastic polyurethane elastomers, poly(methylmethacrylate), poly(butylene terephthalate), polyoxymethylene,polyethylene, poly(vinyl chloride). In particular, the particles have amedian diameter between 0.5 mm and 10 mm, and preferably between 2 mmand 4 mm, and most preferably between 3 mm and 3.5 mm, and have one of agenerally spherical, elliptical, spheroid, elliptoid, cylindrical, orlenticular shape, or combination thereof.

In order to avoid damage of the sensitive filtration membranes by theparticles that are introduced, the roughness of the particles must notexceed a defined measure. According to the invention, particles areused, the surface of which has a median roughness Rtm of less than 40μ,preferably less than 30 μm, and in particular less than 20 μm. Themedian roughness Rtm is determined by taking the median of theroughnesses Rt (DIN EN ISO 4287) of a plurality of particles.

For treating raw water or wastewater or activated sludge, a membranebioreactor system having a filtration pond/tank having at least onesubmerged filtration membrane module is provided. In this case thesystem is distinguished in that the raw water or wastewater or theactivated sludge in the filtration tank contains non-porous,biologically resistant particles.

In an embodiment of the membrane bioreactor, the spacing between twomembranes in the filtration membrane module is up to 8 mm and the mediandiameter of the particles (granules) is less than 5 mm. Advantageously,a feed device for gas, in particular compressed air, is provided for thebottom end of the filtration membrane module, the compressed airstreaming of which moves the particles upward between the membranes. Themaximum specific area loading of the membranes in the filtrationmembrane module is 1 to 80 l/(m²×h). It has been found that thepermeability as a ratio of the specific area loading of the membrane tothe transmembrane pressure in the filtration membrane modules isconstant over an operating time of more than 6 months.

The method achieves the advantages that mechanical erosion of themembrane fouling layers proceeds without additional chemical cleaning,that the flow of the liquid that is to be purified through the membranesremains constant over a time period of several months, abrasive damageto the membrane surfaces by particles occurs only to a very slightextent and the operating costs can thereby be reduced, since theintervals for cleaning the membrane surfaces can be extended.

Also, by means of the method, membranes which already have a foulinglayer, characterized by a very low permeability and high transmembranepressures, can be regenerated again by addition of granules. For thispurpose the membrane module is installed in a cleaning container andparticles that are set in motion are added to the liquid in the cleaningcontainer. The membranes are cleaned within the cleaning container bythe particles. The particles can remain in the cleaning tank and bereused, which gives further cost savings. The cleaned membrane modulescan then again be installed for the filtration operation.

In various embodiments of the present invention, membrane filters, suchas described in U.S. Pat. Nos. 7,892,430 and 5,248,424; U.S. PatentApplication Publication Nos. 2011/0042308, 2011/0042312, 2011/0049038,2011/0127206, 2008/0156730, and 2008/0164208; Japanese Patent Nos.11-128692, H8-155275, and 2951189; and International ApplicationsPCT/EP2009/002944 and WO 2007/036332 (which are incorporated in theirentireties herein by this reference) are enclosed within a waterpermeable housing and within such housing, small particulate beadsand/or pellets are entrained. The enclosure that entrains the beads ispreferably permeable to water so as to permit a flow rate therethrough.In a preferred embodiment, a particular grate material is employed,preferably wedge wire available from the Hendrick Screen Company andsold under the trademark Hendrick Tee screen. A Hendrick Drum screen mayalso be used where the water-current passing the screen is very low.Such a grate material is preferred due to its solid constructioncharacteristics that will withstand the flow pressures within anoperating system while at the same time being very effective to retainbeads within the enclosure without such beads becoming substantiallystuck or lodged within the grate structure itself. Thus, in a preferredembodiment, the enclosure utilized with various embodiments of thepresent invention entrains bead material in such a manner that suchbeads do not substantially get stuck in the enclosure surrounding. Invarious embodiments, a distinct screen material can be employed that hasthe characteristics of permitting water to pass therethrough while alsoentraining enclosed beads that surround a membrane filtration system.The enclosure itself can comprise a housing made from various materials,including metals, composites, plastics, etc. and one of skill in the artwill appreciate the various design modifications for any particularproject in selecting the appropriate enclosure materials to use.Preferably, however, the housing material is made from metal and is ofsufficient strength and anti-corrosion characteristics to persist in anaqueous environment for many years without degradation, corrosion orstructural failure.

In practice, the beads employed to clean the membrane filters are of asize larger than the orifices of the housing enclosure structure so asto restrain such beads within the desired enclosure. The enclosureitself has dimensions suitable to surround at least one membrane filter,and preferably suitable to surround a plurality of membrane filtersprovided in a modular system, such that the housed membrane filter, withor without beads included therein, can be readily and reversibly removedfrom a water treatment environment to facilitate cleaning, retrofitting,modifications, etc. Indeed, in a preferred embodiment, the exteriorenclosure and/or housing of the system includes handles or other contactpoints to permit the removable and moving of the entire enclosure intoand out of a water treatment system. For example, suitable attachmentpoints are provided on the enclosure so that a forklift can be utilizedto engage such contact points and manipulate the movement of the entireenclosure without damage to the delicate membrane filtration systemenclosed therein. Moreover, in various embodiments, there is at leastone removable section of the enclosure to permit not only filtermembranes residing within such enclosure, but the removal of beads usedin the water treatment method, such as after such beads have expiredbeyond their useful life. As such, at least one side of the enclosurecan be reversibly engaged, such as through a conventional latchmechanism so as to permit a hingedly attached portion of the enclosureto be moved to access water membrane filtration modules, replacement ofbeads, and/or to simply permit access to the interior of the enclosurefor any desired purpose.

While the size of beads employed in the present system and method ispreferably the sizes as set forth in U.S. Patent Application PublicationNo. 2011/0042308 to Microdyn-Nadir GmbH or the publication “Beherrschungvon Fouling and Scaling an getauchten Filtrationssystemen inMembranbelebungs-anlagen” [Managing fouling and scaling on submergedfiltration systems in membrane activation systems] by VA TECH WABAGGmbH, Vienna, editor: F. Klegraf. In other embodiments a wider selectionof bead particular sizes is employed in order to address distinct debrisissues that may arise in water treatment facilities. As such, theparticular enclosure materials employed will take into account thesmallest bead size that will be used in order to ensure that themajority, if not all, of the beads employed in such a system areretained within the enclosure when the water filtration system is inuse.

In practice, the enclosure described in the present invention enablesthe water filtration method to be carried out in a fashion so that thevast majority of beads are not freely floating throughout the waterclarifier in which the membrane filters are conventionally employed.Instead, the water clarifier is largely devoid of free floating beads,with such beads being entrained within an enclosure that also enclosesthe water membrane filters.

In various embodiments, more than one of the enclosures is employed in awater treatment system. Such enclosures can themselves be provided in aplurality of design orientations such that at least two and preferablyat least three separate modules are slidably and vertically engageableinto a water clarifier such that water flowing through the clarifierexperiences a series of such enclosures, each enclosure having a waterfiltration filter enclosed therein. Such an aspect of the inventionpermits the use of slightly different membrane filter systems to beemployed in a series. For example, it may be advisable in certainenvironments to have a more porous membrane filter employed initially toremove larger debris and contaminants from water, followed by othermembrane filters along the flow of the water, with such other membranefilters being of a finer quality, thus removing smaller debris andcontaminates. Various membranes and membrane permeability may be used inthe present invention. Such membranes may be of the type similar tothose described in “A Review of Reverse Osmosis Membrane Materials forDesalination—Development to Date and Future Potential” by Lee, et al.published in volume 370 of the Journal of Membrane Science (March 2011),or “Water Permeability and Water/Salt Selectivity Tradeoff in Polymersfor Desalination” by Geise, et al. published in volume 369 of theJournal of Membrane Science (2011). Accordingly, various enclosurescreen sizes together with various bead shapes and sizes can be usedwith the different membrane filters. For example, a very porous membranefilter may be used with an enclosure screen with larger orifices thanthe orifices of the enclosure screen for less porous membrane filter.

In various embodiments of the present invention, a water filtrationssystem is provided, the filtration system comprising a cascade aerationdesign. In one embodiment, a wedgewire or “Hendrick screen” is providedfor removing contaminants. The screen is periodically impacted with acombination of air and/or beads for impinging the surface of the screen.The beads may aid in cleaning the surface of the screen and/or thesurface of the membrane filter. The air may also scour the beads andmove them along the screen and/or membrane filter.

In various embodiments, beads or impingement particles of the presentinvention comprise polypropylene beads. The beads generally comprise adensity greater than water such that they gradually sink when placed inwater. Preferably, beads comprise a density of about 1.05 times that ofwater, such that sinking of the beads occurs slowly. Additionally, asthe beads collect debris from the membrane filter and screen, the beadsmay become more or less dense depending upon the type of debris;therefore the debris should be considered in the design.

As used herein, the term “beads” is meant to generally refer toparticles or devices for impacting filter elements. Accordingly, thisterm should not be read as being limited to any particular size orgeometry of device. It is contemplated that beads of the presentinvention may comprise any number of shapes and/or arrangements and may,but need not be, of a spherical shape.

In various embodiments, wedgewire screens of the present invention areimpacted with an air stream at a small or low angle, such that the airstream can effectively move beads along at least one dimension of thescreen. In this manner, beads may be scoured or moved along the screenin a manner that allows for the beads to perform mechanical cleaningfunctions.

In one embodiment, a pressure differential is applied across a screen inat least one dimension to facilitate migration of beads across thescreen's surface. A pressure differential may be applied across a singledirection. Alternatively, a non-linear pressure differential or aplurality of pressure differentials may be applied over a screen todirect beads and additional cleaning elements across the screen and/orto direct the water across the membranes.

In various embodiments, beads or impingement particles of the presentinvention comprise magnetic beads such that when a magnetic field isinduced upon the system within the enclosure screen and/or thefiltration system, the beads will slowly move along the membrane filterand the screen to mechanically clean the filter and screen.Additionally, when a magnetic field is not induced upon the systemwithin the enclosure screen and/or the filtration system, the beads willrest at the bottom of the filtration tank and thus not impede the waterflow. The speed of the beads, in any direction relative to the membranefilter and screen, can be controlled by the strength of the magneticfield.

In various embodiments, airbursts are periodically used to clean thescreen and/or beads. The airbursts may be applied to a screen inaddition to, or in lieu of, various other cleaning elements of thepresent invention. An airburst system, such a Hendrick Airburst System,is employed in a preferred embodiment.

In an embodiment, the enclosure screen, preferable wedgewire, uses finebubble air scour to clean the screen and/or the membrane filter. In thisembodiment, the cleaning oxygen serves as the mechanical cleaningprocess.

In practice, if a system uses both (1) air or oxygen is used to move thebeads along the screen and membrane or to clean the screen and membraneand (2) the MBR method with biomass to eliminate some pollutants, thenone of the chambers or tanks in the filtration system can be eliminatedbecause aeration and filtration can be performed together in one tank.The air or oxygen is used to move the beads, scour the beads, and cleanthe enclosure screen. Additionally, the air or oxygen aerates the waterin the chamber to replenish oxygen consumed by the biomass. Therefore,separate aeration and filtration chambers are not needed.

The preferred embodiment of the present invention uses vertical filterswhen combining the MBR method with bead- or air-scouring mechanicalcleaning because vertical filters have increased oxygen input andtherefore tend to have higher nitrification, whereas horizontal filtershave decreased oxygen input and are used for denitrification, asdiscussed in the 2008 paper by M. Wichern, C. Lindenblatt, M. Lubken,and H. Horn called “Experimental results and mathematical modeling of anautotrophic and heterotrophic biofilm in a sand filter treating landfillleachate and municipal wastewater,” published in Water Research (42):3899-3909 (incorporated herein by this reference in its entirety).Denitrifying bacteria grow in the anaerobic conditions created deep inthe center of the biofilm, while nitrifying bacteria grow in the outer,aerobic part of the biofilm, as discussed in the 2008 article by L. S.Downing and R. Nerenberg called “Total nitrogen removal in a hybrid,membrane-aerated activated sludge process,” published in Water Research(42): 3697-3708 (incorporated herein by this reference in its entirety).The advantages of the MABR system include energy-saving passiveaeration, reduced tank volume, and the elimination of internal waterrecycling. Challenges found in using such a system include competitionbetween the nitrifying and denitrifying bacteria, which leads to areduction in nitrification and denitrification.

One skilled in the art may combine various aspects of the differentembodiments described herein to make alternate embodiments notspecifically described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the disclosure andtogether with the general description of the disclosure given above andthe detailed description of the drawings given below, serve to explainthe principles of the disclosures.

It should be understood that the drawings are not necessarily to scale.In certain instances, details that are not necessary for anunderstanding of the disclosure or that render other details difficultto perceive may have been omitted. It should be understood, of course,that the disclosure is not necessarily limited to the particularembodiments illustrated herein.

In the drawings:

FIG. 1a shows a filtration tank and a filtration membrane enclosurescreen;

FIG. 1b shows a bead containing housing module and a filtration membranemodule;

FIG. 2a shows a schematic depiction of a membrane bioreactor systemhaving a filtration system similar to the filtration system shown inFIG. 1 a;

FIG. 2b shows a schematic depiction of a membrane bioreactor systemhaving a filtration system similar to the filtration system shown inFIG. 1 b;

FIG. 3 is a perspective view of an embodiment of a filtration assemblyincluding a plurality of vertically-aligned filtration modules alignedalong the lateral edges of individual membrane sheets and an aerationdevice located below the filtration modules;

FIG. 4 shows a diagram of the permeability of the membranes of afiltration membrane system over time;

FIG. 5 shows a diagram of the decrease in permeability of a filtrationmembrane system and the permeability after cleaning;

FIG. 6a shows an enclosure screen or housing material according to oneparticular embodiment of the present disclosure;

FIG. 6b shows a cross-sectional view of a screening or filtering elementof the housing shown in FIG. 6a ; and

FIG. 6c shows a cross-sectional view of another screening or filteringelement of the housing shown in FIG. 6 a.

FIG. 7 shows a table (Table I) that provides criteria forparticle/granule selection in terms of density, diameter, shape andmaterial.

DETAILED DESCRIPTION

FIG. 1a depicts a filtration system 2 a comprising a filtration tank 6 aand an enclosure screen 16 a. As shown, one or more membranes 8, 10 areprovided for continuous filtration of particulates and contaminationsfrom water, for example. In an embodiment, there is a pressuredifferential between the membranes 8, 10 to make the water flow throughthe membranes 8, 10. In an embodiment, the bottom section of theenclosure screen 16 a is a wedgewire screen 18. An air or water stream14, which is passed through the wedgewire screen 18 and along themembranes 8, 10 and the enclosure screen 16 a, comprises a plurality ofbeads 4 for mechanical and/or abrasive cleaning of the enclosure screen16 a and membranes 8, 10. A feed device 12 for gas, in particular forcompressed air, is at the bottom end of or just below the enclosurescreen 16 a. The gas or air bubbles ascending from the feed device 12flow up between the membranes 8, 10 enclosed in the enclosure screen 16a and carry with them the beads 4. After the beads 4 reach the top ofthe membrane 8 or 10, they exit the enclosure screen 16 a and slowlysink down to the bottom of the filtration chamber or tank 6 a by meansof gravity because the beads 4 are slightly denser than water, between1.0 and 1.5 kg/dm³. Preferably the density of the bead material is 1.00to 1.40 kg/dm³, and in particular the density of the bead material has avalue from 1.00 to 1.10 kg/dm³.

In various embodiments, the present invention comprises an enclosure forsurrounding the membrane filters of a filtration system. As shown inFIG. 1a , the enclosure screen 16 a operates as a membrane-housingmodule, such that beads 4 provided in the filtration system 2 a are inclose contact with the membranes and to protect the membranes fromdamage inflicted by large particles (other than the beads 4). In anembodiment the wedgewire screen 18 is at a slight angle to aerate theenclosure screen 16 a and the membranes 8, 10. The wedgewire screen 18can also direct the airstream 14 and the beads 4. The air from the gasfeed device 12 may also use an airburst and/or an airburst system (notshown) to periodically clean the wedgewire screen 18, enclosure screen16 a, membranes 8, 10, and beads 4.

The enclosure screen 16 a is preferably permeable to liquids, such thatwater or fluid may pass through the enclosure screen 16 a substantiallyunobstructed and such that filtration operations are not impeded by thepresence of the novel enclosure. In a preferred embodiment, theenclosure screen 16 a is a wedgewire screen.

To ensure that the enclosure screen 16 a does not substantiallyobstruction the flow of water or fluid nor obstruct the filtrationprocess, the enclosure screen 16 a must be cleaned periodically toremove any build-up on the enclosure screen 16 a.

FIG. 1b depicts a filtration system 2 b comprising a bead containinghousing module 6 b and a filtration membrane module 16 b. In anembodiment, the bead containing housing module 6 b is a birdcage-likescreen. As shown, one or more membranes 8, 10 are provided forcontinuous filtration of particulates and contaminations from water, forexample. An air or water stream 14, which is passed through and/or alongthe membranes 8, 10 and the filtration membrane module 16 b, comprises aplurality of beads 4 for mechanical and/or abrasive cleaning of thefiltration membrane module 16 b and membranes 8, 10. A feed device 12for gas, in particular for compressed air, is at the bottom end of thefiltration membrane module 16 b. The gas or air bubbles ascending fromthe feed device 12 flow up between the membranes 8, 10 of the filtrationmembrane module 16 b and carry with them the beads 4. After the beads 4reach the top of the membrane 8 or 10, they exit the filtration membranemodule 16 b slowly sink down to the bottom of the bead containinghousing module 6 b by means of gravity because the beads 4 are slightlydenser than water, between 1.0 and 1.5 kg/dm³. Preferably the density ofthe bead material is 1.00 to 1.40 kg/dm³, and in particular the densityof the bead material has a value from 1.00 to 1.10 kg/dm³. As the beads4 sink, they mechanically and/or abrasively clean the bead containinghousing module 6 b.

In various embodiments, the present invention comprises an enclosure forsurrounding a filtration system. As shown in FIG. 1b , the enclosureoperates as a bead containment housing module 6 b, such that beads 4provided in the filtration system 2 b are not lost to the outsideenvironment or other portions of the system where they may not berecoverable. The housing module 6 b is preferably permeable to liquids,such that water or fluid may pass through the housing module 6 bsubstantially unobstructed and such that filtration operations are notimpeded by the presence of the novel enclosure. In a preferredembodiment, the housing module 6 b is a screen, preferable a wedgewirescreen.

To ensure that the housing module 6 bb does not substantiallyobstruction the flow of water or fluid nor obstruct the filtrationprocess, the housing module 6 b must be cleaned periodically to removeany build-up on the housing module 6 b.

FIG. 2a shows schematically a membrane bioreactor system 20 a fortreating raw water or wastewater, a denitrification appliance 26, anitrification appliance 28, and a filtration tank 6 a in which aplurality of enclosure screens 16 a are situated. In the denitrificationappliance 26, via a feed line 22, raw water or wastewater is introducedafter it was previously chemically and mechanically pretreated. Inaddition, via a line 24, nutrients pass into the activation stage. Inthe filtration tank 6 a are arranged, for example, five enclosurescreens, of which three enclosure screen modules 16 a are in operation.The enclosure screen modules 16 a are explained in accordance with FIG.1a . These three enclosure screens 16 a are exposed to compressed airvia a feed device 12 for gas, in particular for compressed air, at thebottom end of the respective enclosure screens 16 a. Via a pump, theexcess sludge is transported out of the filtration tank 6 a. The topends of the enclosure screens 16 a are connected to a return line forthe raw water or wastewater. In addition, the water that is purified bythe biologically active material is taken off from the filtration tankby means of a pump in the permeate line.

FIG. 2b shows schematically a membrane bioreactor system 20 b fortreating raw water or wastewater, a denitrification appliance 26, anitrification appliance 28, and a filtration tank 30 in which aplurality of enclosure screens, also called bead containing housingmodules, 6 b are situated. In the denitrification appliance 26, via afeed line 22, raw water or wastewater is introduced after it waspreviously chemically and mechanically pretreated. In addition, via aline 24, nutrients pass into the activation stage. In the filtrationtank 30 are arranged, for example, five bead containing housing modules,of which three bead containing housing modules 6 b are in operation. Thebead containing housing modules 6 b are explained in accordance withFIG. 1b . These three bead containing housing modules 6 b are exposed tocompressed air via a feed device 12 for gas, in particular forcompressed air, at the bottom end of the respective bead containinghousing modules 6 b. Via a pump, the excess sludge is transported out ofthe filtration tank 30. The top ends of the bead containing housingmodules 6 b are connected to a return line for the raw water orwastewater. In addition, the water that is purified by the biologicallyactive material is taken off from the filtration tank 30 by means of apump in the permeate line.

FIG. 3 illustrates an embodiment of a filtration assembly 46 generallyshown, submerged in a body of feed water which is subject to ambientpressure, such as a pond or open tank. The filtration assembly 46comprises a plurality of filtration modules positioned in side-by-sidearrangement with spacing between vertically-aligned membrane sheets 8.The average spacing between the surfaces of membrane sheets 8,preferably from about 2 to 12 mm, defines a fluid flow pathway generallyindicated by upwardly pointing arrows. More preferably, the averagespacing between the surfaces of the membrane sheets 8 are from 3 to 7mm, which must be slightly larger than the diameter of the cleaningbeads. In other embodiments, the average spacing is less than 6 mm andin some embodiments less than 4 mm. The spacing between adjacentmembrane sheets is preferably uniform, i.e. deviating from the spacingat the header by less than 50% and more preferably less than 25%. Aswill be subsequently described, the fluid flow pathway is unconfinedalong the top edges of the individual membrane sheets 8. Permeateoutlets 48 extending from terminal headers on each end of the assemblyprovide routes for transferring permeate from the filtration modules. Inone preferred embodiment, the permeate outlet 48 is in fluidcommunication with a pump (not shown) which creates negative pressure(vacuum), and which draws permeate from the headers. The negativepressure is communicated to the outer surfaces of the membrane sheetsand creates a transmembrane pressure necessary for filtration. That is,negative pressure generated by a pump creates a transmembrane pressurewhich induces flow of permeate through the porous structure of themembrane sheet, to the chambers of individual headers, through thepermeate outlet 48 where permeate can then be collected, stored or used.By reversing the pressure generated by the pump, or by use of a separatepump, stored permeate may be backwashed through the filtration assembly.

The filtration assembly may optionally include an aeration device 50located below the filtration modules for delivering gas bubbles 52generated by an external pump and gas source (not shown) into the feedsource. The gas (preferably air) bubbles are delivered to the feedsource by a series of pipes 54 with apertures 56 or nozzles. As thebubbles 52 exit the apertures 56, they rise vertically within the feedsource along the fluid flow pathway defined by the spacing betweenvertically-aligned membrane sheets. As the bubbles 52 pass along thefluid flow pathway, they effectively scrub the outer surfaces of themembrane sheets and at least partially remove accumulated solids fromthe porous structure of the membrane sheets. The bubbles 52 may alsocarry beads (not shown) to mechanically and/or abrasively clean thefiltration membrane modules and membranes 8, 10.

Although the description herein is made particularly to an aerobicmembrane bioreactor, it is expressly understood that the embodimentsdescribed herein may work with an anaerobic membrane bioreactor as well.Other membrane types and filtration devices may be cleaned using thesystem and method of the present disclosure, in its varying embodiment.

The filtration assembly preferably has a relatively high packingdensity. More specifically, the assembly preferably has a membranespecific surface area of at least 150/m, and in some embodiments atleast 200/m. For purposes of the present description, the term “specificsurface area” means the active membrane area of the assembly per unitvolume. The “active membrane area” means the outer surfaces of themembrane which are porous and in fluid communication with the capillarychannels. Thus, the use of non-porous laminates, support edges andreinforcing strips are excluded from the “active membrane area”. Thevolume of the assembly includes the region within the edges of themembranes sheets (located between the terminal membrane sheets at eachend of the assembly). Thus, the volume includes the spacing betweenindividual membrane sheets 8. The use of the present membrane sheetsallows for closer spacing of membrane sheets.

FIG. 4 shows the permeability of all enclosure screens 16 a or beadcontaining housing modules 6 b over time. All modules began with apermeability in the range from 400 to 500 l/(m²*h*bar) (initialpermeability 100%). It may clearly be seen that the permeability remainsconstant over a period of several months in the enclosure screens 16 aor bead containing housing modules 6 b with PP granules. Thepermeability in the enclosure screens 16 a or bead containing housingmodules 6 b during the experimental phase reduced in the course of 2.5months to about 40% of the initial permeability. By means of weeklyin-situ cleaning, the permeability could readily be increased to about50% of its initial permeability.

By adding about 0.5 to 1.0 kg/m³ of granules in the reference train, thecleaning action of the granules could be demonstrated. In the course offour days, the permeability increased to its initial state (FIG. 4).

FIG. 5 shows the result of a cleaning Cleaning of membranes alreadybearing a fouling layer is possible. A filtration membrane module thathad a permeability of only 20% of its initial permeability was treatedwith air in a cleaning tank with water and an addition of 1 to 10 kg/m³,in particular about 3 to 5 kg/m³, of granules over about 10 to 14 hoursin such a manner that the particles circulated. After completion ofcleaning, the module was again put in operation and exhibited itsinitial permeability.

Referring now to FIGS. 6a-6c , one enclosure screen material in apreferred embodiment is shown. FIG. 6a shows a wedgewire screen having aplurality of supports, which support a plurality of wires spaced apartto form a plurality of regularly spaced orifices. The plurality of wiresare preferably spaced apart so as to contain substantially all of theplurality of non-porous biologically resistant particles within thehousing enclosure.

As shown in FIG. 6c , the wires are preferably triangular in shape andoriented so that the smallest leg of the triangle faces towards theinterior of the housing enclosure (i.e., FIG. 6a depicts the enclosurescreen material in a top view, the top view being the side facinginwardly when assembled to surround the membrane modules). Thisorientation ensures that any non-porous biologically resistant particlesthat inadvertently escape through the wedgewire screen do not becometrapped between the wires, thereby impeding the circulation of liquidthrough the wedgewire screen and any pressure differential createdthrough the housing enclosure. Other shaped wires may achieve this sameresult without departing from the spirit of the disclosure.

Although the present disclosure is described as a system and method forprimarily treating a membrane bioreactor system, it is expresslyunderstood that other types of filtration apparatus may be cleaned usingthe systems and methods described herein. For example, the filtrationsystems described in U.S. Pat. Nos. 7,435,351, 7,223,247, 7,143,781,which are incorporated by reference herein in their entireties, areconsidered within the scope of the present disclosure and may be used inconjunction with the systems and methods of the present application.

Although not shown in the drawing figures, the particles that aredescribed herein are important for the system and method described inthis disclosure. The particles are preferably selected from the groupconsisting of polypropylene—mineral filled or mineralparticle-containing, polycarbonate blends, thermoplastic polyurethaneelastomers (TPE), poly(methyl methacrylate), poly(butyleneterephthalate), polyoxymethylene, polyethylene, poly(vinyl chloride).The particles should have a median diameter between 0.5 mm and 10 mm,and preferably between 2 mm and 4 mm, and most preferably between 3 mmand 3.5 mm. The particles preferably have one of a generally spherical,elliptical, spheroid, elliptoid, cylindrical, or lenticular shape, orcombination thereof. The preferred density of the particles is between1.0 g/ml and 1.10 g/ml, and most preferably 1.05 g/ml.

It was found that in the method for treating raw water or wastewater byusing particles in the filtration tanks, the control of the formation ofmembrane fouling layers is reinforced. It was found that the beneficialeffect of the particles used and the basic function of treating the rawwater or wastewater which is to separate the biomass and purified waterfrom one another is achieved. The costs of the chemical cleaning can bereduced by the method according to the invention. Likewise, the effecton the environment is less lasting, since fewer chemicals are liberatedand therefore the potential for byproducts such as, for example,absorbable organic halogen compounds, is reduced.

While various embodiment of the present disclosure have been describedin detail, it is apparent that modifications and alterations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and alterations are withinthe scope and spirit of the present disclosure, as set forth in thefollowing claims. For further illustration, the description of theclaimed invention as encompassed in the Claims appended hereto areexpressly made a part of this disclosure and incorporated by referenceherein in their entirety.

The foregoing discussion of the disclosure has been presented forpurposes of illustration and description. The foregoing is not intendedto limit the disclosure to the form or forms disclosed herein. In theforegoing Detailed Description for example, various features of thedisclosure are grouped together in one or more embodiments for thepurpose of streamlining the disclosure. This method of disclosure is notto be interpreted as reflecting an intention that the claimed disclosurerequires more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the followingclaims are hereby incorporated into this Detailed Description, with eachclaim standing on its own as a separate preferred embodiment of thedisclosure.

Moreover, though the present disclosure has included description of oneor more embodiments and certain variations and modifications, othervariations and modifications are within the scope of the disclosure,e.g., as may be within the skill and knowledge of those in the art,after understanding the present disclosure. It is intended to obtainrights which include alternative embodiments to the extent permitted,including alternate, interchangeable and/or equivalent structures,functions, ranges or steps to those claimed, whether or not suchalternate, interchangeable and/or equivalent structures, functions,ranges or steps are disclosed herein, and without intending to publiclydedicate any patentable subject matter.

What is claimed is:
 1. A method for cleaning filtration membranemodules, comprising: providing at least two removable housingstructures, with each of said at least two removable housing structureshaving a wedgewire screen that entirely surrounds one or more filtrationmembranes on all four sides and on top and on bottom, said at least tworemovable housing structures comprising at least four verticallyextending panels on each side of said one or more filtration membranesand a horizontally extending top and a horizontally extending bottom,said removable housing structure, adapted to be positioned within abasin for containing wastewater; positioning said at least two removablehousing structures in said basin; flushing the one or more filtrationmembranes with a liquid containing biologically inert particles;circulating the biologically inert particles through the liquid and in amanner to achieve contact between the biologically inert particles andthe one or more filtration membranes, said biologically inert particlesimpacting both sides of said one or more filtration membranessimultaneously during said circulating step; wherein deposits situatedon one or more surfaces of the one or more filtration membranes aremechanically abraded by the biologically inert particles, and whereinthe biologically inert particles are of a size able to be retainedwithin the at least two removable housing structures; wherein duringsaid circulating step, said biologically inert particles impact each ofthe four sides and top and bottom of the at least two removable housingstructures, and wherein said wedgewire screen has openings that permitfluid to flow therethrough, said at least two removable housingstructures being devoid of any opening through which said biologicallyinert particles can escape during said circulating step.
 2. The methodof claim 1 wherein the biologically inert particles are non-porous andgenerally ellipsoid in shape and have a smallest diameter ofapproximately 2 mm to 4 mm.
 3. The method of claim 1 wherein the step ofcirculating is achieved by a pressure differential applied across theone or more filtration membranes and within the at least two removablehousing structures.
 4. The method of claim 1 wherein a pressuredifferential is applied across the at least two removable housingstructures.
 5. The method of claim 1 wherein a pressure differential isapplied across the one or more filtration membranes to facilitatemigration of said biologically inert particles across a surface of saidone or more filtration membranes.
 6. A system for cleaning a filtrationmembrane module, comprising at least two removable housing structuresthat each include a filtration membrane, each of said at least tworemovable housing structures completely surrounding said filtrationmembrane, each of said at least two removable housing structures havinga wedgewire screen on each of four sides, top and bottom of saidremovable housing structures, said removable housing structurescontaining a plurality of biologically inert particles that when set incirculation adjacent the filtration membrane, mechanically abradedeposits situated on outer surfaces of the filtration membrane, whereinthe biologically inert particles are of a size larger than regularlyspaced orifices of the wedgewire screen so as to restrain thebiologically inert particles within the housing structures; and whereinthe removable housing structures are devoid of any opening through whichsaid biologically inert particles can escape.
 7. The system of claim 6wherein the biologically inert particles are non-porous and generallyellipsoid in shape and are approximately 2 mm to 4 mm in diameter. 8.The system of claim 6 wherein the biologically inert particles areformed of a mineral filled polypropylene.
 9. The system of claim 8wherein the biologically inert particles are non-porous and have adensity between 1.0 g/ml and 1.10 g/ml.
 10. The system of claim 6wherein the biologically inert particles are set in circulation by apressure differential applied across the filtration membrane module. 11.The system of claim 10 wherein the pressure differential is appliedacross a single direction.
 12. The system of claim 10 wherein thepressure differential is applied across the filtration membrane tofacilitate migration of said biologically inert particles across asurface of said filtration membrane.
 13. The system of claim 6 whereinthe biologically inert particles are shaped to abrade deposits situatedon outer surfaces of the filtration membrane without abrading orremoving the surface of the filtration membrane itself.
 14. The methodas set forth in claim 1, wherein said removable housing structuresoperate as a containment module that prevents biologically inertparticles from being lost to the environment outside to the removablehousing structures.
 15. The method as set forth in claim 1, wherein saidremovable housing structures surround a single one of the one or morefiltration membranes.
 16. The method as set forth in claim 1, whereinsaid one or more filtration membranes are reversibly removable from awater treatment environment to facilitate cleaning.
 17. The method asset forth in claim 1, wherein said removable housing structures comprisehandles to permit the removal of said one or more filtration membranesinto and out of a water treatment system.
 18. The method as set forth inclaim 1, wherein said removable housing structures comprise a forkliftattachment utilized to engage and manipulate said removable housingstructures without damage to the one or more filtration membranes housedtherein.
 19. The method as set forth in claim 1, wherein at least oneside of said removable housing structures is reversibly engaged and isreversibly secured with a conventional latch mechanism to permit accessto said one or more filtration membranes housed within the removablehousing structures.
 20. The method as set forth in claim 1, wherein thebiologically inert particles have a median roughness of less than 40 μm.21. The method as set forth in claim 1, wherein the biologically inertparticles have a median roughness of less than 30 μm.
 22. The method asset forth in claim 1, wherein the biologically inert particles have amedian roughness of less than 20 μm.
 23. The method as set forth inclaim 1, wherein the biologically inert particles consist of inertpolymeric material.
 24. The method as set forth in claim 1, wherein thebiologically inert particles comprise a material having a densitygreater than water.